spirv: Move SPIR-V building to Makefile.spirv.am and spirv/meson.build

Future changes will add generated files used only from
src/compiler/glsl.  These can't be built from Makefile.nir.am, and we
can't move all the rules from Makefile.nir.am to Makefile.spirv.am (and
it would be silly anyway).

v2: Do it for meson too.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (the meson bits)
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com> (the automake bits)

src/compiler/meson.build

@@ -21,6 +21,7 @@
 inc_compiler = include_directories('.')
 inc_nir = include_directories('nir')
 inc_glsl = include_directories('glsl')
+inc_spirv = include_directories('spirv')
 
 files_libcompiler = files(
   'blob.c',
@@ -53,8 +54,12 @@ libcompiler = static_library(
   build_by_default : false,
 )
 
+subdir('spirv')
+
 subdir('nir')
 
+# This is here because putting it in spirv/meson.build would create a circular
+# dependency with nir/meson.build.
 spirv2nir = executable(
   'spirv2nir',
   [files('spirv/spirv2nir.c'), dummy_cpp],